MIPS: Delete definition of SA_RESTORER.

SA_RESTORER used to be defined as 0x04000000 but only the O32 ABI ever
supported its use and no libc was using it, so the entire sa-restorer
functionality was removed with lmo commit 39bffc12c3580ab [Zap sa_restorer.]
for 2.5.48 retaining only the SA_RESTORER definition as a reminder to avoid
accidental reuse of the mask bit.

Upstream cdef9602fbf1871a43f0f1b5cea10dd0f275167d [signal: always clear
sa_restorer on execve] adds code that assumes sa_sigaction has an
sa_restorer field, if SA_RESTORER is defined which would break MIPS.
So remove the SA_RESTORER definition before the v3.8.4 merge.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
(cherry picked from commit 17da8d63add23830892ac4dc2cbb3b5d4ffb79a8)